The present invention relates to thermal barriers for use between the pumps and motors of glandless pump and motor assemblies. More particularly, the invention relates to improvements in thermal barriers which can be utilized in submersible pump and motor assemblies for circulation of liquid media which are maintained at elevated pressures and temperatures. Such assemblies are often utilized in fossil fuel power stations as well as in nuclear power plants. For example, a glandless pump and motor assembly can be used to circulate feedwater at a temperature of up to and in excess of 375.degree. C. Moreover, such an assembly is often used as a pressure vessel for supercritical feedwater at a temperature of up to 450.degree. C. and a pressure ranging up to and even in excess of 375 bars.
When a pump and motor assembly is used at temperatures within the aforementioned ranges, it is necessary to shield the motor from excessive temperatures of the liquid which is circulated by the pump. Such function is performed by the thermal barrier which is interposed between the casing of the pump and the housing of the motor and serves to prevent the liquid which flows into the housing of the motor from reaching or remaining at temperatures which could cause damage to the motor. The thermal barrier constitutes a mechanical connector between the pump and the motor as well as a shield for the motor insofar as the temperature of the liquid entering the motor is concerned. In other words, a satisfactory thermal barrier must establish a rigid mechanical connection between the casing of the pump and the housing of the motor which drives the pump shaft, and the thermal barrier must further shield the motor from excessive temperatures by ensuring that the liquid which is permitted to flow from the pump casing into the motor housing is maintained at a temperature which is invariably below a preselected value. Moreover, a satisfactory thermal barrier must act not unlike a rigid coupling between the pump and the motor so as to ensure that the shaft which receives torque from the motor is maintained in an optimum position with reference to the pump casing. The thermal barrier also snields the motor from excessive pressures of the fluid which surrounds the assembly if the latter is used for circulation of feedwater in a nuclear power plant or the like.
In accordance with one of many presently known proposals, the thermal barrier is designed as an elongated neck-like insert between the pump casing and the motor housing so as to provide a long path for the transfer of heat from the hot pump to the cold or cooler motor. On the other hand, U.S. Pat. No. 3,947,154 discloses a relatively short lantern-shaped thermal barrier which is provided with ribs and is cooled by air and/or water. As a rule, the thermal barrier is connected to the pump casing and to tne motor housing by means of tie rods, bolts and/or screws so as to establish and maintain a rigid connection between the pump and the motor.
Relatively long thermal barriers exhibit several serious drawbacks. Thus, the long intermediate portion of the pump shaft (namely, that portion which is surrounded by the relatively long thermal barrier) is likely to vibrate unless the dimensions of the shaft are increased for the express purpose of eliminating or reducing vibration. This entails additional initial costs and contributes to the bulk and weight of the assembly.
The proposal which is disclosed in the aforementioned U.S. Patent exhibits the drawback that the component parts of the thermal barrier are highly complex, and hence expensive, castings or welded components. The cost of the thermal barrier which is disclosed in the U.S. Patent is particularly high if the thermal barrier embodies the liquid cooling feature. Cooling with a liquid entails the development of pronounced pressure peaks in the component parts.
British Pat. No. 869,506 discloses a thermal barrier which consists of welded-together components. This device exhibits the drawbacks that its initial and maintenance costs are extremely high, that its components cannot be readily tested for the presence or absence of defects (especially in the regions of welded seams), and that the connection between the pump and the motor does not exhibit a satisfactory stability.